Hydrophobic filters are used in filtration of gases, in venting filters, and as gas vents. These hydrophobic filters allow gases and vapors to pass through the filter while liquid water is repelled by the filter.
Polytetrafluoroethylene (PTFE) has been the most commonly used material in filters for gas venting. PTFE is chemically and biologically inert, has high stability, and is hydrophobic. PTFE filters therefore allow gases to be selectively vented while being impervious to liquid water.
Hydrophobic membranes are used as filters in healthcare and related industries, for example, as vent filters for intravenous (IV) fluids and other medical devices. In the healthcare industry, the membrane is sterilized before use. PTFE membranes can be sterilized for these health-related applications with steam or by chemical sterilization without losing integrity.
Treating PTFE membranes with steam can cause pore blockage due to condensation of oil from the machinery used to generate the steam. The resulting loss of air permeability reduces the membrane's ability to serve as an air vent. Although chemical sterilization minimizes exposure of the membrane to oil, chemical sterilization uses toxic chemicals and can generate byproducts, which cause waste disposal problems. Ionizing radiation has also been used for sterilization of materials used in medical and biological devices. PTFE may become unstable when exposed to ionizing radiation. Irradiated PTFE membranes have greatly reduced mechanical strength and cannot be used in applications where they are subjected to even moderate pressures.
Perhaps the two biggest drawbacks to PTFE as a filter for venting gases are the high cost and the low air permeability of PTFE membranes. PTFE membranes are formed by extruding and stretching PTFE. Processing methods to form PTFE membranes may be expensive. Furthermore, the extruding and stretching processes used to form PTFE membranes create a membrane which has relatively, low air permeability.
As a result, efforts have been made to identify alternative substrates which are less expensive and have higher air permeability than PTFE and which can be modified to be hydrophobic.
Coating filtration substrates allows one to retain the desirable bulk properties of the substrate while only altering the surface and interfacial properties of the substrate. Coating substrates to increase their hydrophobic properties has not been very practical, because the coatings can reduce permeability. Furthermore, many of the coating materials are expensive.
Scarmoutzos (U.S. Pat. No. 5,217,802) modified the surface of substrates made of nylon, polyvinylidene difluoride (PVDF), and cellulose by treating the substrate with a fluorinated acrylate monomer. The substrate was sandwiched between two sheets of polyethylene, and the monomer was polymerized by exposing to ultraviolet light. The resulting composite filters had hydrophobic and oleophobic surfaces. The air permeability of the filters decreases with time.
Moya (U.S. Pat. No. 5,554,414) formed composite filters from polyethersulfone and PVDF membranes with a method similar to that of Scarmoutzos. The resulting filters did not wet with water or hexane. The disadvantage of the Moya filters is that air permeability of the treated filters was lower than the untreated substrates, and the fluorinated monomer is expensive.
Sugiyama et al. (U.S. Pat. No. 5,462,586) treated nylon fabric and PTFE membranes with solutions containing two different preformed fluoropolymers. The treated filters were resistant to water and oils. The durability of filters coated with preformed polymers is often less than that for filters where the coating is formed by polymerizing a monomer on the surface of the substrate, however.
Kenigsberg et al. (U.S. Pat. No. 5,156,780) treated a variety of membranes and fabrics with solutions of fluoroacrylate monomers and formed coatings on the substrate by polymerizing the monomer. The coating conferred oil and water repellency onto the substrate. However, the airflow through the treated membrane was reduced, compared to the untreated membrane.
Hydrophobic media suitable for garments have been made by extruding mixtures of polypropylene or PTFE and the fluorochemical oxazolidinone as disclosed in U.S. Pat. No. 5,260,360. The media made by extrusion tend to have relatively low air permeability.
In copending U.S. application Ser. No. 09/323,709 filed Jun. 1, 1999 (incorporated herein by reference in its entirety), oleophobic and hydrophobic filters are prepared by forming a polydimethylsiloxane coating on a polymeric substrate by polymerizing vinyl terminated siloxane with a crosslinker such as hydrosilicon in the presence of a catalyst.
In copending U.S. application Ser. No. 09/778,630 filed Feb. 7, 2001 (incorporated herein by reference in its entirety), oleophobic and hydrophobic filters are prepared by forming a fluorosulfone oligomer coating on a substrate, such as a hydrophobic or hydrophilic membrane or other filtration medium.
In copending Australian Application Number PR5843 filed Jun. 20, 2001 (incorporated herein by reference in its entirety), hollow fiber membranes for use in microfiltration are prepared from a terpolymer of tetrafluoroethylene, vinylidene fluoride, and hexafluoropropylene.
There is a need for a hydrophobic filter, which is inexpensive and has high air permeability. Specifically, there is a need for a filter medium that is hydrophobic and that may be readily and reproducibly produced through simple casting solution phase inversion processes.